1. Field of the Invention
The invention relates to an arrangement comprising a phase-locked loop, whose signal path incorporates a phase detector, a loop filter and a controlled oscillator, and a pilot generator for generating a local auxiliary pilot which is applied to the controlled oscillator, and a pilot detector an input of which is coupled to the signal path of the loop and an output of which is coupled to a level detector via a filter member.
2. Description of the Related Art
An arrangement of this type is known per se from Japanese Patent publication Kokai 63-107304 and supplies a binary output signal which gives an indication about the state of phase lock of the phase-locked loop.
In the phase-locked state of the loop, the output signal of the controlled oscillator is frequency-modulated with the local auxiliary pilot. In the known arrangement, this frequency-modulated oscillator signal is applied to a frequency discriminator operating as a pilot detector. To this end, an input of the known pilot detector is coupled to the signal path between the output of the controlled oscillator and an input of the phase detector. The auxiliary pilot demodulated by the frequency discriminator is applied to the level detector via a bandpass selection in the filter member. The level detector supplies the above-mentioned binary output signal which has the value of 0 or 1 with respect to a given threshold value in dependence upon the amplitude of the auxiliary pilot selected by means of the filter member.
In the phase-locked state of the loop, the oscillator frequency corresponds to the frequency to which the frequency discriminator is tuned, i.e. the zero frequency, and the frequency-modulated oscillator frequency varies within the detection range of the frequency discriminator. The frequency discriminator then supplies the local auxiliary pilot at an amplitude which exceeds said threshold value and thus gives rise to a given output signal value of the level detector, for example 1.
Outside the locking range, the oscillator frequency deviates from said zero frequency to such an extent that it does not coincide with the detection range of the frequency discriminator and that the auxiliary pilot cannot be detected. Consequently, the frequency discriminator supplies no or substantially no signal component at the frequency of the auxiliary pilot so that the output signal of the bandpass filter member remains below said threshold value and the level detector supplies the output signal value of 0. For a correct locking indication, the dimensioning of the frequency discriminator and hence the location, the steepness and the range of the detection curve, as well as said threshold value of the level detector should be accurately set to correct values with respect to the locking range of the phase-locked loop. The circuit settings required for this purpose are complex and time-consuming, hence costly.